Influence of day and night temperature differentials on root elongation rate, root hydraulic properties, and shoot water relations in chrysanthemum

The difference between night and day temperature (DIF = day - night tempera ture) has been shown to affect plant height. A positive DIF (+DIF), cooler night than day temperature, increases stem elongation while a negative DIF (DIF), warmer night than day temperature, decreases stem elongation. The ph ysiological mechanism underlying the growth response to DIF is not understo od, however, and the effects of day/night temperature differentials on root permeability to mater and root elongation rate have not been studied. The objective of this study was to describe how +DIF and -DIF temperature regim es affect leaf water relations, root water flux (J(v)), root hydraulic cond uctivity (L-p), and root elongation rates of 'Boaldi' chrysanthemum [Dendra nthema xgrandiflora Kitam,'Boaldi' (syn, Chrysanthemum xmorifolium Ramat.)] plants over time. Leaf turgor pressure (psi p) was 0.1 to 0.2 MPa higher i n plants grown in a +6 degrees C DIF environment throughout both the Light and dark periods, relative to those in a -6 degrees C DIF environment, J(v) differed markedly in roots of plants grown in +DIF vs -DIF environments, R hythmic diurnal patterns of J(v) were observed in all DIF treatments, but t he relative timing of flux minima and maxima differed among treatments. Pla nts grown in positive DIF regimes exhibited maximum root flux at the beginn ing of the light period, while those in negative DIF environments had maxim um root nux during the first felv hours of the dark period. Plants grown in +DIF had significantly higher L-p than -DIF plants. Plants grown in +DIF a nd -DIF environments showed differences in the diurnal rhythm of root elong ation. During the dark period, +DIF plants exhibited minimal root elongatio n rates, while -DIF plants exhibited maximal rates. During the light period , the converse was observed. In -DIF temperature regimes, periods of rapid root elongation coincided with periods of high J(v). Results of this study suggest that negative DIF environments lead to leaf turgor reductions and m arkedly alter diurnal patterns of root elongation These changes may, in tur n, act to reduce stem elongation.